1. Field of the Invention
The present invention relates to a thin film field effect transistor and a display using the same. Particularly, it relates to a thin film field effect transistor in which an amorphous oxide semiconductor is used for an active layer, and a display using the same.
2. Description of the Related Art
In recent years, flat panel displays (FPDs) have been put to practical use, due to the progress made in liquid crystal and electroluminescence (EL) technologies, etc. Especially, an organic electroluminescence element (hereinafter referred to as an “organic EL element” in some cases) formed using a thin film material which emits light by excitation due to application of electric current can provide light emission of high brightness at low voltage, and thus is expected to achieve reduction in device thickness, weight, and size, and power saving, etc. in wide ranging applications including mobile phone displays, personal digital assistants (PDA), computer displays, car information displays, TV monitors, and general illumination.
These FPDs are driven by an active matrix circuit including thin film field effect transistors each using, as an active layer, an amorphous silicon thin film or a polycrystalline silicon thin film provided on a glass substrate. (In the description below, the thin film field effect transistor is sometimes referred to as a “thin film transistor” or “TFT”.)
On the other hand, to make the FPD thinner, lighter, and more resistant to breakage, attempts are being made to use a resin substrate which is light in weight and flexible instead of the glass substrate.
However, fabrication of the transistors using the silicon thin films described above requires a thermal treatment process at a relatively high temperature, and it is difficult to form the transistors directly on a resin substrate which is generally low in heat resistance.
For example, in Japanese Patent Application Laid-Open (JP-A) No. 2006-121029, a MOSFET (Metal-Oxide Semiconductor Field Effect Transistor) that reduces a drive voltage of a transistor that uses a silicon thin film is disclosed, and a configuration that uses indium tin oxide (ITO), tin oxide or zinc oxide as a semiconductor material of an active layer and uses a dielectric material having a large dielectric constant in a gate insulating layer is disclosed. It is disclosed that ITO, tin oxide or zinc oxide is a crystalline metal oxide and has a carrier concentration of substantially 1×1019 cm−3. In the case of an active layer made of the crystalline metal oxide, in order to obtain desired semiconductor characteristics, after film formation by sputtering, a high temperature heat treatment step such as post-annealing at 300° C. for 15 min is necessary in order to control the crystallization (see, for example, paragraph No. [0054] of JP-A No. 2006-121029). Accordingly, such an active layer is difficult to form directly on a resin substrate that is poor in heat resistance.
An amorphous oxide such as an In—Ga—Zn—O-based amorphous oxide can form a film at low temperatures, and, accordingly, has been attracting attention as a material capable of forming a film at room temperature on a plastic film (see, for example, NATURE, vol. 432, pages 488-492, Nov. 25, 2004). However, when an amorphous oxide semiconductor is used in an active layer of a TFT, an OFF current is high, and accordingly, there is a problem in that an ON/OFF ratio is low. For example, APPLIED PHYSICS LETTERS, 89, 062103 (2006) discloses a configuration of a TFT that uses IZO (In—Zn—O) having a carrier concentration of 2.1×1017 cm−3 in an active layer as an amorphous oxide semiconductor. However, since the OFF current is still high, a “normally-on” state is caused, in which a current flows even in a state where a TFT gate voltage is not applied. Accordingly, there is a problem in that an extraneous circuit for an ON/OFF operation is necessary and power consumption increases.
As means for solving the problem, JP-A Nos. 2006-165529 and 2006-186319 disclose that, when the carrier concentration of an amorphous oxide semiconductor is reduced to, for example, less than 1018 cm−3, a TFT operates, that when the carrier concentration is less than 1016 cm−3, a TFT having excellent ON/OFF ratio is obtained, and that, in order to impart more excellent low off current characteristics, the carrier concentration is preferably reduced to less than 1016 cm−3.
However, in TFTs supplied for practical use, in addition to low OFF current and high ON/OFF ratio, it is required that the characteristics do not vary even under continuous driving and stable performance is exhibited even when conditions such as a temperature or humidity in an operating environment vary. That is, there still remain many problems to be overcome.